1. Field of the Invention
The present invention generally relates to silver nanostructures and a method for producing the same. The invention relates to synthesis of silver nanoparticles and nanowires suitable for large-scale production and use in interconnects of fine circuit pattern, more specifically, and forming conductive adhesives for use in electronic parts, such as electrode of light emitted diode, electrode packing on PCB substrate and packaging materials for integrated circuits.
2. Description of the Prior Art
For fundamental research, when the size of solid reaches nanometer order, the solid's property will be vastly changed due to the small size effect and quantum effect. For metal, especially the noble metal (here we refer to silver (Ag) and gold (Au)), their optical (surface plasma effect) and catalytic properties are mostly dominated by their morphology. Many research works at present are aiming at exploring and utilizing these unique characters[Ref. 1-5], which promotes the manufacturing of silver or gold nanostructures in large scale.
From technique and industrial point of view, silver powders are widely used in electronic industry, especially used to fabricate conductive adhesives. The conductive adhesive is used in electronic parts, such as electrode of light emitted diode, electrode packing on PCB substrate and packaging materials for integrated circuits. The conventional conductive adhesive is produced by mixing conventional adhesive and a conductive power such as carbon, copper, aluminum or silver powder. Silver is known to have higher electronic and thermal conductivity, but be much lower in price than that of other noble metals like gold and platinum. These characters make silver powder a competitive candidate as a filler filled in conductive adhesive. In most cases, high concentration (about 75˜85% in weight) of silver particles or flake like silver plates in size from micrometer to sub-micrometer are used to form a conductive pattern in the adhesive to have a low resistivity at the order of 10−4 ohm-cm. A problem coming with this technique is that such a high concentration of filler will dramatically degrade the properties of adhesive itself, eg. its shear strength, impact strength. To lower the filler concentration meanwhile without compromising its conductivity, therefore, is always a challenge. Dr. Wu.HP[Ref. 6] (in paper “High conductivity of isotropic conductive adhesives filled with silver nanowires” International Journal of Adhesion & Adhesives 26 (2006), 617˜621) demonstrated a combination of silver particles and nanowires would drastically lower the threshold of filling concentration to reach a high conductivity. An even lower filling concentration could be expected reasonably if only nanowires are used in the adhesives. This brings a challenge on how to produce silver nanowires selectively in a scalable fashion.
Known methods of manufacturing silver nanowire are broadly divided into template-assistant and template-free methods. For template-assistant methods, porous AAO (Anodic Alumina Oxide), carbon nanotube and some “soft-template” like porous TiO2 are widely used as a template that will induce silver to grow into nanowires. The morphology of the nanowires such as diameter and wire length can be controlled when choosing different templates. For template-free methods, silver particles or silver nanowires are produced by reducing it from silver salt at temperature 80˜150° C.; morphology of which can be tailored by tuning the amount of surfactant, or adding different reducing agent[Ref. 7-21].
Papers and Patents listed at the end of this section can be viewed as references.                [1] Plasmonic material. W. Andrew Murray and William L. Barnes. Adv. Mater. 19 (2007), 3771        [2] Microchannel networks for nanowire patterning. Benjamin Messer et al., J. Am. Chem. Soc. 122 (2000), 10232        [3] Optical trapping and integration of semiconductor nanowire assemblies in water. Peter J. et al. Nature Materials, 5, 97        [4] General synthesis of compound semiconductor nanowires. X.Duan and C M Lieber. Adv. Mater. 12 (2000), 298        [5] Polarization-dependent surface-enhanced raman scattering from a silver nanoparticle decorated single silver nanowire. Seung Joon Lee, et. al. NanoLett. 8 (2008), 3244        [6] High conductivity of isotropic conductive adhesives filled with silver nanowires. H. P. Wu, et. al. International Journal of Adhesion & Adhesives 26 (2006) 617-621        [7] Silicalite-1 Hollow Spheres and Bodies with a Regular System of Macrocavities, Valentin Valtchev, Chem. Material., 14 (2002), 4376        [8] Evidence for the Monolayer Assembly of Poly(vinylpyrrolidone) on the Surfaces of Silver Nanowires, Y. Gao et. al J.Phys.Chem.B, 108 (2004), 12877        [9] Facile synthesis of Ag nanocubes and Au nanocages, Skrabalak et al, Nature Protocols 2 (2007), 2182        [10] Trimeric clusters of silver in aqueous AgNO3 solutions and their role as nuclei in forming triangular nanoplates of silver., Xiong, Y et al, Angewandte Chemie International Edition 46 (2007), 4917        [11] Synthesis and optical properties of silver nanobars and nanorice. Wiley, B. J. et al, Nano Letters 7 (2007), 1032        [12] Synthesis and Electrical Characterization of Silver Nanobeams. Wiley, B et al. Nano Lett. 6 (2007), 2273        [13] Rapid Synthesis of Small Silver Nanocubes by Mediating Polyol Reduction with a Trace Amount of Sodium Sulfide or Sodium Hydrosulfide. Siekkinen, A. R et al. Chem. Phys. Lett. 432 (2006), 491        [14] One-Dimensional Nanostructures: Synthesis, Characterization, and Applications. Y xia. Et al. Adv. Mater. 15 (2003), 353        [15] Reduction by the End Groups of Poly(vinyl pyrrolidone): A New and Versatile Route to the Kinetically Controlled Synthesis of Ag Triangular Nanoplates. Washio, Y et al. Adv. Mater. 18 (2003), 1745.        [16] Capillarity and silver nanowire formation observed in single walled carbon nanotubes Jeremy Sloan et. al. Chem. Commun., 699 (1999), 700        [17] Ultrathin Single-Crystalline Silver Nanowire Arrays Formed in an Ambient Solution Phase, Byung Hee Hong et. al, Science, 294 (2001), 348        [18] U.S. patent application Ser. No. 11/906,950 filed Oct. 3, 2007.        [19] U.S. patent application Ser. No. 11/045,974 filed Aug. 9, 2007.        [20] U.S. patent application Ser. No. 11/736,036 filed Aug. 9, 2007.        [21] Convenient, Rapid Synthesis of Ag Nanowires et. al, Chem. Mater. 19 (2007), 1755        
Silver nanowires obtained by template-assisted method are always impure, since it is hard to retrieve silver from its templates. For example, the silver covered Ag-carbon-nanotube (CNY) composite nanowires, the carbon nanotube functioned as substrate is difficult to remove; and the coverage of silver is incomplete and imperfect at most of times. In addition, the template itself such as AAO has its drawback, which is fragile, small in size and difficult to produce.
On the other hand, template-free method is the widest used method to synthesis silver particles and nanowires in laboratory, where Prof. Xia. YN in the University of Washington had developed it a lot since 1990s, who used a double-inject apparatus that allowed silver salt to be reduced to silver and grow into nanowire simultaneously. Nevertheless, disadvantages of this wet-chemistry method are still obvious, due to its low manufacturing efficiency. Usually, hours to days of time are needed, but only milligrams of outputs will get from the manufacturing procedure. This is also the problem for template-assistant method.
Coprecipitation of silver salt and precipitator is another method to get large volume of silver particle which is a commonly used method in industry. The outputs are plate-like or particle like silver ranging from micrometer to sub-micrometer in size; no nanowire or nanoparticle could be got.
Recently, a microwave-assisted solution method is used as a rapid synthesis to get silver nanowire [21], however, the efficiency is still at a low level, which yields mixture of nanoparticle and wires with only 0.04 kg silver per litre volume of solvent per reaction time in hour.
Therefore, an object of the present invention is to overcome such problems by promoting a method which could produce silver nanowires and nanoparticles with high efficiency. FIG. 1 is a comparison between this patent results and previous reports, in terms of kilograms silver produced per unit of liter*hour. We demonstrate that the disclosed method can produce silver nanostructures selectively and the processing efficiency (Kg/(L*h)) to produce silver is at least one order of magnitude higher than the best published results. Another object is to fabricate a prototype of conductive adhesive utilizing selectively synthesized silver nanowires.